An LED array is used as a light source of an image scanning apparatus. However, if one LED array irradiates a document surface, since irradiation width on the document surface is narrow, a shadow of an image tends to appear when a level difference portion of an original document is scanned. Since a light amount of the one LED array is small, if a more light amount is necessary, a method of increasing a current value or changing LEDs to high-power LEDs to increase the light amount is adopted. However, in such a method, a ripple of illuminance tends to occur on the document surface. Therefore, it is conceivable to increase the irradiation width and increase the light amount by using two LED arrays.
The LED arrays are mounted on a component surface of a substrate. However, it is likely that, if LEDs are kept on for a long time, a temperature difference occurs between the component surface and a solder surface of the substrate because of a temperature rise of the LEDs alone and the substrate warps. If the substrate warps, an illuminance characteristic of the document surface is adversely affected.